The Use of a Microcontrollers in Hygrometric Measurements Application

Abstract The article presents the possibility of using microcontroller systems as one of elements of a teaching position. The Arduino Mega system based on the ATMega 2560 microcontroller from the AVR family was used to build the station. At the beginning, a virtual hygrometer model was designed in the AUTOCAD program and then the air channels were made using 3D printing. After assembling the station, it was compared to a laboratory aspiration hygrometer. The analysis was presented in the final part of the article.


INTRODUCTION
Already at the turn of the fifteenth and sixteenth centuries, attempts were made to study the change of water vapor content in the air. The first primitive hygrometers were based on the method of hygroscopic measurement, e.g. change in the weight of a woolen sphere absorbing moisture from the atmosphere. These studies contributed to the further development of this field, and other solutions and materials that absorb moisture have been sought [2,4].

MEASUREMENT OF AIR HUMIDITY USING A HYGROMETER
The psychrometer is a device from the family of hygrometers. The simplest consists of two thermometers, one of which is surrounded by a fabric soaked with distilled water, and shows temperature of the wet thermometer. And the other is an ordinary thermometer indicating the ambient temperature [1].
The measurement in the psychrometric method depends on the state of saturation with the steam of the air flowing around the wet thermometer. The result of the difference in component pressures of the water vapor (air flowing around the wet thermometer) and the ambient air is the evaporation of water from the so-called hygroscopic. The tank with the thermometric liquid cools to the temperature corresponding to the cooling limit [1].
The psychometric difference is difference between indication of a dry and wet thermometer. It depends on the humidity of ambient air. The lower relative humidity, greater temperature difference. If the air is saturated, namely φ = 100%, both thermometers will show the same temperature [1].
We use the Sprung formula for psychrometric calculations. It is based on heat taken from the atmosphere used to evaporate the water from wet bulb thermometer. This heat returns in the form of heat of evaporation of water into the air.
With the assumed conditions of heat exchange and free inflow of air mass, knowing the temperatures of thermometers dry air temperature) and (wet air temperature), we can determine the exact air humidity for t s ≤ 323,15K, using the Sprung formula [1]. where: = (65 + 6.75 ) • 10 −5 • −1 .
A -psychrometric constant dependent on factors such as the type of gas, gas velocity around a container with thermometric liquid thermometer, wet bulb temperature. Psychrometer construction, dimensions and shape of a thermometric liquid container have a very big influence on the psychrometric constant [1].
The Assmann psychrometer is a device consisting of two dry and wet thermometers surrounded by cover ( fig. 1). It is additionally equipped with a reservoir with thermometric liquid protection against influence of solar radiation and a fan that generates forced airflow at a speed w of 2

THE METHOD OF CALCULATING THE RELATIVE HUMIDITY
The rapid development of microcontrollers was the reason that these systems are increasingly used in various projects, including those related to temperature measurement. Cheap electronic circuits can be used as replacements for expensive components. Projects have already been carried out in which infrared thermometers [5,6] and microcontrollers [3] were used for hygroscopic measurements. Due to the fact that there is a gap in the availability of simple tools for hygroscopic measurements, authors decided to use DS18B20 digital thermometers for temperature measurement using the Arduino Mega microcontroller system with an LCD display.

ASPIRATION PSYCHROMETER USING A MICROCONTROLLER SYSTEM
The creation of microcontroller systems forced the appearance of ready devices using these systems. The development of IDE software and libraries has made programming of these electronic components much easier. One of the examples are Arduino systems, which in their design use microcontrollers from the AVR family. The Arduino Mega 2560 R3 was selected for the construction of the psychrometer. The board uses the AVR ATmega 2560 microcontroller. Fig. 2 shows the MEGA2560 R3 board. Dallas DS18B20 digital thermometers were used to measure the temperature of dry and wet parts. For communication, they use the 1-Wire protocol. Fig. 3 shows the temperature sensor DS18B20. DC electrical characteristics shows tab. 1.   fig. 4. The plate was made using the thermal transfer method and etched with sodium persulfate.
To make the body of the psychrometer on which the fan was installed, 3D printer was used. For this purpose, a virtual model of the psychrometer was designed in AutoCad program and later the G code was constructed on it. 3D printing was carried out on an IPrus printer. The stand, on which the air flow ducts together with the fan were installed, was made of larch wood. Fig. 5 shows the visualization of the hygrometer model in the AutoCad program.
The system was programmed in the Arduino IDEand mounted on the platform. Fig. 6 shows the complete aspiration psychrometer with the control and measurement system. Scientific Journal of PNA

RESEARCH AND CALCULATION
The designed aspiration psychrometer was compared with a psychrometer recognized as the model company of 'WSZ Kraków'. The test consisted in measuring the temperature of the 'wet' and 'dry' thermometers of both psychrometers. Eight Scientific Journal of PNA measurements were carried out -four under present atmospheric conditions and four using steam produced by an electric kettle. After analyzing the measurement results, the test was presented using tables and diagrams. Fig. 7 The temperature results of both psychrometers, which were obtained during measurements under atmospheric conditions, were similar to each other. The relative values calculated on their basis differed from 0.16 [%] to 1.55 [%] depending on the measurement points. Therefore, an arithmetic mean was derived from them, which amounted to 0.69 [%]. This is a favorable result for the digital psychrometer compared to its model counterpart.
In summary, microcontroller systems can be used to build teaching positions. Together with digital sensors that measure to two decimal places, they are an alternative to this type of device.
The position in the future can be equipped with a sensor measuring atmospheric pressure. This would allow a more accurate reading of relative humidity and would improve the work of this device.